$\alpha$ handles the conversion between the units in which labor is measured and the the units in which consumption is measured.
Consider the leisure part of the utility function:
$$ - \alpha \frac{n^{1 + \frac{1}{\nu}}}{1 + \frac{1}{\nu}}$$
We can rewrite it like this:
$$ - (\alpha^{\frac{1}{1 + \frac{1}{\nu}}})^{1 + \frac{1}{\nu}} \frac{n^{1 + \frac{1}{\nu}}}{1 + \frac{1}{\nu}} = - \frac{(n\cdot (\alpha^{\frac{1}{1 + \frac{1}{\nu}}}))^{1 + \frac{1}{\nu}}}{1 + \frac{1}{\nu}} $$
Define $\gamma = (\alpha^{\frac{1}{1 + \frac{1}{\nu}}})$
and rewrite the utility function in terms of $\gamma$ instead of $\alpha$
$$ u(c, n) = \log (c) - \frac{(\gamma \cdot n)^{1 + \frac{1}{\nu}}}{1 + \frac{1}{\nu}}$$
Written this way, I see $\gamma$ and therefore $\alpha$ as serving as a unit shifter. How many seconds of leisure makes you indifferent over one dollar less consumption? Say it were 60 seconds. What unit is $n$ measured in? Should we put in $n=1$ for minutes, $n=60$ for seconds, or $n=\frac{1}{60}$ for hours? $\nu$ can't answer that question because it controls the shape and curvature but not the scale of the utility function with respect to $n$. However, $\gamma$ can. If the "true" measure of $n$ is in hours and we guess wrong and choose seconds then $\gamma$ should be $\frac{1}{3600}$ and conditional on $\nu$ this implies $\alpha$.
In calibration the true units of $n$ it need not map cleanly to one of our time units. $n$ might be properly measured in units of 15.5 seconds or $\pi$ minutes, but the functional form is flexible enough to handle these cases regardless of if you input the labor in the calibration in any of the more standard units of hours, minutes, or seconds.
As for how to calibrate this, you have to see the bundles of labor/ leisure and consumption chosen by households for given sets of prices. This, combined with the demand functions that follow from this utility setup, should give you the required calibration. The paper A Model of Housing in the Presence of Adjustment Costs: A Structural Interpretation of Habit Persistence (Flavin and Nakagawa (2008), free copy here) shows the way to do this in a different (housing / non-housing consumption instead of consumption / leisure) problem. Their $\gamma$ parameter acts much like $\alpha$ does here.